It is known that magnetic particles (‘beads’) embedded in a liquid can be used to carry a probe molecule on their surface that specifically interacts with a complementary target molecule (for example single stranded probe DNA interacting with complementary target DNA). Upon reaction with a molecule to be probed and, for example, using optical or electrochemical measurements, one can determine the amount of target molecules on a bead or within a certain volume containing beads. The interest in using magnetic beads, is that they can be manipulated using magnetic fields irrespective of fluid motion. In this way one can create an important relative motion of the beads with respect to the fluid and, hence, a large probability of binding a target molecule to a probe molecule fixed on the bead surface. One can then magnetically extract the beads to a place of detection/collection. Historically, beads have been locally fixed by using external magnets or have been transported using mechanically moving external magnets. The latter procedure may be for example used to fabricate mixing devices and in immuno-assay methods.
Here and in the following, particles smaller than 100 microns are considered, which are often also called beads. The beads typically have a size in the range between 0.1 and 50 microns, e.g. in the range of 1 micron.
“Separation” of magnetic beads means that a liquid flow, containing the beads, passes a zone with a large magnetic field (gradient) and that the magnetic beads are filtered out (separated) by the field. Magnetic transport of beads is essential for bringing the beads to a well-defined position within a micro fluidic circuit, for example near to a magnetic bead detection device. “Transport” means that the beads are effectively moved by a magnetic force, i.e. using a magnetic field and not just retained by a magnetic field from a liquid solution passing by (=separation). Nevertheless, manipulation of these beads in general and transport in particular, is a difficult task, as the effective relative magnetic susceptibility of the (super)paramagnetic beads is rather weak (typically <<1, due to demagnetization effects of the mostly spherical particles) and the magnetic volume of the particles is small. This explains why mostly the large field of (mechanically moving) permanent magnets or large electromagnets have been used for the separation, transport, and positioning of magnetic beads. In other work, micropatterned conductors, actuated by large currents, have been demonstrated to present a useful solution for magnetic beads capture and transport. These devices allow precise positioning and transport over 10-100 μm distances in a single actuation event.
US 2005/284817 A1 discloses a device for transporting magnetic or magnetizable beads in a capillary chamber comprising a permanent magnet or an electromagnet for subjecting the capillary chamber to a substantially uniform magnetic field, to apply a permanent magnetic moment to the beads. At least one planar coil and preferably an array of overlapping coils are located adjacent to the capillary chamber for applying a complementary magnetic field on the beads parallel or antiparallel to said substantially uniform magnetic field, to drive the beads. An arrangement is provided for switching the current applied to the coil(s) to invert the field produced thereby, to selectively apply an attractive or repulsive driving force on the beads. The device is usable to transport beads for performing chemical and biochemical reactions or assay, as is done for instance in clinical chemistry assays for medical diagnostic purposes.
Since the NIH (National Health Institute) project to initiate the sequencing of the whole human genome at the end of the 1990's, the technological developments in sequencing technology have been going very rapidly. Especially since the introduction of 2nd generation of sequencing machines by 454 Life Sciences (now Roche) in 2005 (see M. Margulies, M. Egholm et al., Nature, 437 (2005) 376-380) developments have intensified. Currently, a number of other companies also have launched 2nd gen. sequencing machines, and it is the desire to reduce the costs of DNA sequencing further so that DNA sequencing will become a clinical tool in the analysis of, for instance, cancer.
One of the general strategies to reduce cost further is to miniaturize the sequencing devices, in particular by integration of the steps that are necessary for sequencing in a micro-fluidics device. In such an approach, the DNA to be sequenced as well as the reagents involved in the sequencing reactions, are manipulated within micro-channels and chambers of sub-millimeter dimensions. The manipulation can be done in various ways, such as with micro-pumps and valves, integrated micro-actuators, electrokinetic driving forces, magnetic driving forces, or by exploiting surface tension.
In some of the next-generation sequencing approaches, magnetic micro-beads are used a substrates for the DNA strands to be sequenced. In particular, ideally each single bead has one unique DNA strand attached to it, that is copied millions times on the same bead (using PCR). Typically, for multiplying the same strands many more times on a single bead in order to increase single to noise ratio emulsion bead PCR multiplications (emPCR) are used. When miniaturizing such an approach, it would be very advantageous to be able to manipulate the beads in a controlled way, using magnetic fields generated locally in the device. This would offer the opportunity to transport beads with specific strands attached to it to particular locations in the device, while monitoring their exact position.